Buildings can be constructed to be air tight to reduce the amount of conditioned (e.g., heated or cooled) air escaping the building or to reduce the amount of outside air infiltrating into the building. Mechanical ventilation can be added to bring in fresh air to improve air quality within the building. This fresh air can be brought into the building using a central fan and an air duct system associated with a central heating system and/or an air conditioning system to inject an amount of fresh air from an outside known source and distribute it throughout the home. A controller such as that included in the AirCycler™ ventilation system available from Lipidex Corporation of Marshfield, Mass. can facilitate this process. The AirCycler™ ventilation system can monitor a thermostat to determine a duration of time during which the thermostat causes the central fan to bring in fresh air (e.g., as part of heating or cooling). If the amount of fresh air brought in that duration of time is less than a target amount, the AirCycler™ ventilation system can turn on the central fan for an additional time period to bring in an additional amount of fresh air to meet the target amount.